Positioning information of a mobile station may be used for many purposes:                pricing of calls may be based on the position of a mobile station, whereby calls made from the home area, for example, may be cheaper;        when an emergency call is placed from a mobile station, it is possible to determine the position of the mobile station (this feature will be required by the authorities in some countries);        the user of a mobile station may need information about his/her location e.g. when traveling,        the authorities can use the positioning information to locate a stolen mobile station or to trace a missing person, for example.        
As position determination involves data communication in the system, it is necessary that the operator be able to charge for the location service according to use. In addition, positioning has to be reliable so that the location information cannot be falsified by the user e.g. in order to be allowed a home area discount while elsewhere than in the home area. Furthermore, the data may be confidential as it is not necessarily in the interests of the operator to reveal system-related information to a third party.
The Global Positioning System (GPS) based on the utilization of signals received from satellites is widely used for positioning purposes. This solution would require that a GPS receiver be added to mobile stations, which would result in considerable additional costs. Therefore, to locate a mobile station in a positioning system intended for all mobile phones it is preferable to utilize the cellular system's own signals transferred between the mobile station and base stations, even though some mobile station models may also include a GPS receiver.
A method is known from publication [1] WO 92/05672 wherein the distance of a mobile station from a base station is determined on the basis of the propagation delay of a signal transferred between the mobile station and the base station. By measuring at the base station the propagation delay of the signal sent by the mobile station, it is obtained a distance estimate for the range between the mobile station and the base station. Because of a certain measuring accuracy associated with the distance measurement, the estimated location of the mobile station is a ring-shaped area the width of which depends on the accuracy of the time delay measurement. A corresponding propagation delay measurement may also be performed for a signal between the mobile station and other base stations. The result is then one ring-shaped location area per each base station involved in the measurement. Thus, the mobile station can be located at the intersection of the location areas, the dimensions of the intersection corresponding to the measuring accuracy. In the method described above, position determination is substantially carried out by the system, and no special functions are needed in mobile stations to realize the measurement.
A disadvantage of the method based on the propagation delay measured from a signal between a mobile station and base stations is that the propagation delay cannot be easily measured with such an accuracy that the accuracy of the mobile station location information would be adequate for all the above-mentioned applications. In addition, the mobile station has to establish an uplink connection with every base station for which the propagation delay is to be measured. If the positioning information has to be continuously updated, the data traffic associated with the positioning puts a considerable load on the connection capacity of the mobile communication system. Furthermore, the high volume of data traffic makes the measurement slow. Another problem with the method is that errors in the absolute timing accuracy of the mobile station result in errors in the positioning result obtained.
It is also known a measurement based on the time difference of received signals, wherein the measurement can be performed by the mobile station or by the system. Let a mobile station receive signals from a plurality of base stations. An estimate for the position of the mobile station is determined by means of the observed time difference (OTD) between the signals received from the base stations, whereby it is possible to calculate by means of the time difference between signals received from two base stations the difference d1-d2 between the distance d1 between the mobile station and a first base station and the distance d2 between the mobile station and a second base station. Then those potential location points of the mobile station in which the value of the distance difference equals d1-d2 constitute a hyperbola-shaped curve, which thus represents the potential location points of the mobile station. Since the measurement result involves a certain error margin, the location area of the mobile station is in reality a band between two hyperbolas, the width of the band depending on the error margin of the measurement result. When signals are received from at least three base stations, the result consists of a plurality of location areas and the mobile station is located at the intersection of those areas. Determining a restricted location area requires time difference measurement for signals received from at least three base stations unless other methods such as propagation delay measurement are used in addition to the time difference measurement. If other additional methods are used, it is possible to use the time difference measured for signals received from only two base stations. Such determination of location of a mobile station can be realized either by the mobile station or by the system.
From patent document [2] FI 101445 it is known a solution in which a mobile station measures the time differences of signals received from base stations and sends the measured time difference data to the mobile communication system. A mobile location center in the mobile communication system uses the measured time difference data, base station position coordinates and base station relative time difference (RTD) data to calculate the location of the mobile station.
In the solutions disclosed in documents [1] and [2], the location of a mobile station is calculated by the cellular network. A solution is known from document [3] EP 398773 wherein a mobile station receives from a mobile communication system information about the relative time differences and position coordinates of base stations situated in the area around the mobile station. The mobile station then measures the time differences of the signals received from said base stations and determines the position of the mobile station on the basis of the measured time differences and the base stations' relative time differences and position coordinates, using a position determination algorithm stored in the mobile station.
So, the time difference between signals received by a mobile station from two base stations is called an observed time difference (OTD). Observed time difference measurement is used for synchronizing the mobile station with the clock of the new serving base station in connection with a hand-off, so the OTD measuring method is known in the prior art. The OTD is measured in two steps because a mobile station cannot receive signals from two base stations simultaneously. First the mobile station measures a first time difference between the mobile station's own clock and the clock of the signal received from a first base station. Then the mobile station measures a second time difference between the mobile station and a second base station. The observed time difference OTD is obtained by calculating the difference of the first and second time differences.
In mobile communication systems, such as e.g. GSM and UMTS, the lengths of the control messages sent to mobile stations are fixed and the channels through which control messages are sent have limited capacities. When implementing new services, such as e.g. the positioning services described above, there often emerges a need to convey more information than what can be included in the message reserved for that particular purpose. For example, the SMS-CB (Short Message Service Cell Broadcast) message may be used to convey positioning data, but this message is rather short, only 88 octets of which an 82-octet payload portion may be used to transfer positioning data. As was mentioned above, the positioning methods involve a lot of communication between a mobile station and the network. For example, base station location data are typically indicated using WGS-84 coordinates, so one message cannot include location data for very many base stations. One solution would be to send more messages and thereby convey more information, but this would spend the limited resources of the control channel in question. The transmission interval of SMS-CB messages, for example, is two seconds, so the limited communication capacity results in considerable delays if more than one message is needed to convey one data set. Thus it is needed a better solution for this problem.